A common technique for compacting powdered material consists in forming the mould space in a so-called die bolster and in producing the compact by means of an upper ram and a lower ram. Usually, the lower ram is moved into the die bore up to a predetermined position, whereupon filling is effected with powdered material. Subsequently, the compact is formed with the aid of the upper ram. Such a technique is used, for instance, to compact metallic powder for the manufacture of molded components according to the sintering process. This aims at molding the compact in a relatively precise way already, if possible, with a view to its geometrical dimensions and its density so as to achieve the desired dimensional accuracy later after the sintering process.
If the compact has a geometrical shape in which an oblique surface is provided at the outside as is the case, for instance, in cutting blades for milling and drilling tools a very significant deformation force is applied to the die bolster during the compaction procedure. The deformation force causes the die bolster to get deformed by flexing and upsetting. The flexing effect thus caused on the bolster may be reduced by a skilful selection of the supporting surfaces and the die bolster cross-sections, but cannot be eliminated.
In the compaction process described, the deformation of the die bolster may not be ignored. It is necessary for the upper ram to travel to and stop at the edge at the transition point of the mould surfaces in a precise manner. If the upper ram is not stopped at this point the ram and die bolster will be damaged. On the contrary, there will be a lack of dimensional accuracy if the ram is stopped too early.
It is known to determine by tests or calculations by which amount a die bolster undergoes deformation in a certain compaction procedure to predetermine the displacement length of the upper ram. This is normally accomplished by ascertaining on the compact whether or not the compaction ram has traveled through the predetermined distance. Such a technique involves relatively great expenditure and does not protect the compaction device from damage. If relatively low compacting forces occur because material was insufficiently filled in there will be no de-formation of the die bolster or it achieves distinctly smaller values so that if the compaction ram is positioned the upper ram will strike against the edge of the bolster bore, as a consequence.
It is the object of the invention to provide a process for compacting powdered material by which a compact may be manufactured in a reproducibly precise manner while protecting the compaction device against unintended damage caused by insufficient die bolster deformation.
The invention relies on the fact that the flexing force acting on the die bolster results from the difference of the compacting forces applied by the upper and lower rams. In the inventive process, a curve or table is obtained to report the dependence of die bolster deformation from the compacting forces applied. Furthermore, to adjust the feed paths of the compaction, it is essential to know which displacements of the die bore occur if deformations differ. Therefore, in the inventive process, the compacting forces are measured from time to time or even continuously during the compaction process to determine the respective deformation. A certain deformation rate of the die bolster also includes a predetermined feed path for the compaction rams. Therefore, it is possible to correct the length of the feed length by means of the inventive process during the compaction procedure depending on the results of the measurements described. Therefore, an outcome of the invention is that the upper ram is precisely moved up to the edge of the die bore without touching it significantly, however.
When the die bolster is deformed a relative displacement of the lower ram and the die bore will also occur naturally. Hence, it is necessary to correct the feed length of the lower ram concurrently with the correction described for the feed length of the upper ram.
The inventive process allows to prevent the upper ram from striking against an edge of the die bore if no deformation occurs to the die bolster. Since the compacting force is consistently measured as was mentioned, but can also fall below certain values this way permits to determine the time the entire compaction device needs to be stopped to avoid damage to both the upper ram and die bore.
In the compaction process described, not only does the die bolster undergo deformation, but the upper and lower rams also undergo an upsetting deformation. The deformation rates are relatively small as is the deformation of the die bolster, but are not negligible. Thus, for instance, a deformation of some xcexcm per tonne of compacting force is obtained in a die bolster. To enable a correction also in the event of a non-negligible upsetting of the compaction rams, an aspect of the invention provides that the upsetting deformation of the rams are measured or calculated for various compacting forces thereon. The correlating values of the upsetting deformation and compacting forces are filed as a table in a memory. Then, the feed rate of the upper and lower rams will be corrected depending on the extent of upsetting.